Certain technological methods, particularly procedures involving biological materials, involve the use of cryogenic temperatures. For example, for stabilizing biological specimens for later microscopic examination, the specimens are often quickly frozen and the water in the samples is then exchanged or replaced with suitable organic solvents such as acetone at low temperatures. To reduce damage to the specimens, the substitution of an organic liquid for water often takes place at temperatures in the range of from about -80.degree. to about -120.degree. C., and this reduced temperature must be maintained over a period of time. As taught in U.S. Pat. No. 4,306,425 (Sitte, et al), the time required for such substitution may range from about three days to about three weeks. Various procedures in the field of genetic engineering require specimens to be subjected to cryogenic temperatures. For certain biological procedures, the time rate of cooling is of importance. Cryogenic temperatures may be used for storage of various biological materials such as heart valves, corneas, bone marrow cells, skin tissue, spermatozoa, etc.
Various devices have been proposed for enabling the temperature of a biological specimen or other material to be controllably lowered and maintained. Certain of such devices have employed, in combination, a cryogenic liquid or vapor to cool a specimen and a heater such as an electric resistance heater to heat the specimen, the specimen temperature being controlled, to the extent possible, by varying the amount of heat energy supplied to the specimen holder. Reference is made, for example, to the above-mentioned U.S. Pat. No. 4,306,425. It also has been proposed to suspend a specimen within a dewar flask above the level of liquid cryogen, and to raise and lower the specimen as desired to obtain the desired temperature or cooling rate.
The devices that utilize a cryogen for cooling a specimen in opposition to a heater for warming the specimen are commonly difficult to control. In one such device, for example, a cryogenic vapor is sprayed or evaporated into a compartment having electric heaters in its walls, the compartment having an interior fan for the purpose of equalizing the temperature throughout the compartment. This device is difficult to regulate with any degree of accuracy, since a large amount of time is required for the compartment to come to a steady state temperature, or to be varied from one temperature to another; that is, the response time of the apparatus is quite large. The positioning of a specimen at given levels within a dewar flask similarly is difficult to control, since the temperature gradient extending from the level of the liquid cryogen to the mouth of the flask generally is greatly compressed near the liquid level and accordingly the temperature varies greatly within a short space above the liquid level.